10KΩ
20KΩ
10KΩ
20KΩ
10KΩ
20KΩ
R
I
FEEDBACK
V
REF
I
REF
20KΩ
R = 10KΩ
20KΩ
S
0
S1
S2
S11
R = 10KΩ
V
REF
OUT1
I
I
1/4096
85pF
30pF
LEAKAGE
I
I
OUT2
OUT1
I
OUT2
R
FB
LEAKAGE
D11 (MSB)
D1
D2
D0 (LSB)
Switches shown for digital inputs "high"
Figure 3. Equivalent Circuit – All Inputs High
Figure 1. Simplified DAC Circuit
2001V ESD Protection
The twelve output current-steering switches are
in series with the R-2R ladder, and therefore,
can introduce bit errors. It is essential then, that
the switch “on” resistance be binarily scaled so
that the voltage drop across each switch remains
constant. If, for example, switch S0 of Figure 1
was designed with an “on” resistance of 10
ohms, switch S1 for 20 ohms, etc., then with a
10V reference input, the current through S0 is
0.5mA, S1 is 0.25mA, etc.; a constant 5mV drop
will then be maintained across each switch.
In the design of the HS7541A’s data inputs,
2001V ESD resistance has been incorporated
through careful layout and the inclusion of input
protection circuitry.
Equivalent Circuit Analysis
Figures2and3showtheequivalentcircuitsforall
digital inputs LOW and HIGH respectively. The
referencecurrentisswitchedtoIOUT2 whenallinputs
are LOW, and to IOUT1 when all inputs are HIGH.
The ILEAKAGE current source is the combination of
surface and junction leakages to the substrate; the
1/4096currentsourcerepresentstheconstant1-bit
current drain through the ladder terminating resis-
tor. The output capacitance is dependent upon the
digitalinputcode,andthereforevariesbetweenthe
low and high values.
To further insure accuracy across the full tem-
peraturerange,permanently“on”MOSswitches
are included in series with the feedback resistor
andtheR-2Rladder’sterminatingresistor.These
series switches are equivalently scaled to two
times switch S11 (MSB) and to switch S0 (LSB)
respectively to maintain constant relative volt-
age drops with varying temperature. During any
testing of the resistor ladder or RFB (such as
incoming inspection), VDD must be present to
turn “on” these series switches.
Output Impedance
Theoutputresistance,asinthecaseoftheoutput
capacitance, varies with the digital input code.
The resistance, looking back into the IOUT1 ter-
GAIN TRIM
+15V
R
I
FEEDBACK
V
(–10V)
REF
2KΩ
17
16
R = 10KΩ
4
GAIN TRIM
D
(MSB)
V
V
18
1
11
REF
DD
R
FB
OUT1
+15V
15pF
1KΩ
I
I
LEAKAGE
30pF
85pF
I
O1
–
+
I
V
REF
INPUT
DATA
OUT
HS7541A
R = 10KΩ
V
I
REF
OUT2
–15V
15
2
1/4096
LEAKAGE
I
D
(LSB)
O2
0
GND
3
Figure 2. Equivalent Circuit – All Inputs Low
Figure 4. Unipolar Operation
HS7541A
12-Bit CMOS Multiplying DAC
© Copyright 2000 Sipex Corporation
5
SIPEX [ SIPEX CORPORATION ]
